In recent years, an internal combustion engine for a vehicle is provided with an exhaust gas purifying system. The exhaust gas purifying system includes a catalyst, which is for purifying exhaust gas in an exhaust pipe, and exhaust gas sensors such as an air-fuel ratio sensor or an oxygen sensor for detecting an air-fuel ratio or a rich-lean state of exhaust gas. The exhaust gas sensors are provided respectively on the upstream side and on the downstream side of the catalyst. The air-fuel ratio is feedback controlled on the basis of the outputs of the exhaust gas sensors so as to enhance efficiency of purification of exhaust gas by using the catalyst.
In such an exhaust gas purifying system, the exhaust gas sensors need a malfunction diagnosis so as to avoid continuous operation of the system in a state where the exhaust gas sensors are deteriorated to decrease in accuracy of the air-fuel ratio control and to decrease in efficiency of purification of the exhaust gas. Generally, the output of an exhaust gas sensor (catalyst downstream sensor) provided on the downstream side of a catalyst is exerted with an effect of the quantity of oxygen as the quantity of lean component that the catalyst adsorbs. Therefore, the output of the exhaust gas sensor tends to delay with respect to a change in an air-fuel ratio on the upstream side of the catalyst.
In consideration of the foregoing characteristics, U.S. Pat. No. 6,374,818 B2 (JP-A-2001-215205) proposes an apparatus configured to determine a malfunction of a catalyst downstream sensor during a fuel cut of an internal combustion engine or after the fuel cut, i.e., after starting of fuel injection. In the present apparatus, when a predetermined time elapses after starting the fuel cut (fuel cut start), it is determined whether the catalyst downstream sensor is normal on the basis of determination whether the output of the catalyst downstream sensor becomes leaner than a threshold. In the case where the determination of malfunction of the catalyst downstream sensor is not performed during the fuel cut, thereafter, it is determined whether the catalyst downstream sensor is normal on the basis of determination whether the output of the catalyst downstream sensor becomes richer than a threshold after a predetermined time subsequent to the fuel cut. At this time, if a summation of the quantity of exhaust gas after the fuel cut is smaller than a predetermined value, the determination of malfunction of the catalyst downstream sensor is suspended even when the output of the catalyst downstream sensor is leaner than the threshold after the predetermined time subsequent to the fuel cut. The present operation avoids erroneous determination of malfunction of the catalyst downstream sensor in the state where change in air-fuel ratio of the exhaust gas on the downstream side of the catalyst is delayed due to decrease in summation of exhaust gas subsequent to the fuel cut. That is, erroneous determination can be avoided even when the quantity of exhaust gas as a rich component decreases, and thereby change in air-fuel ratio delays to cause change in output of the catalyst downstream sensor.
However, the time between the fuel cut start and change in output of the catalyst downstream sensor includes a dead time. Accordingly, in U.S. Pat. No. 6,374,818, the responsivity of the catalyst downstream sensor cannot be evaluated with high accuracy. Consequently, reduction in responsivity of the catalyst downstream sensor cannot be detected with high accuracy.